Optical reflectors for spectrometer gas cells

ABSTRACT

A spectrometer cell can include a spacer, at least one end cap, and at least one mirror with a reflective surface. The end cap can be positioned proximate to a first contact end of the spacer such that the end cap and spacer at least partially enclose an internal volume of the spectrometer cell. The mirror can be secured in place by a mechanical attachment that includes attachment materials that are chemically inert to at least one reactive gas compound. The mechanical attachment can hold an optical axis of the reflective surface in a fixed orientation relative to other components of the spectrometer cell and or a spectrometer device that comprises the spectrometer cell. The mirror can optionally be constructed of a material such as stainless steel, ceramic, or the like. Related methods, articles of manufacture, systems, and the like are described.

TECHNICAL FIELD

The subject matter described herein relates generally to opticalreflectors, and more particularly in at least some implementations tooptical reflectors usable in spectroscopic instruments for analysis ofgas mixtures.

BACKGROUND

Spectroscopic analyzers and other instruments or equipment that relyupon measurements of absorption or emission of light (herein referred togenerally as “spectrometers”) can be used in a broad range ofapplications for determining the presence and amount of one or moretarget analytes in a gas volume or flowing gas stream. Somespectrometers include a spectrometer cell, which can generally featureone or more optical components (e.g. windows or mirrors) for admittinglight from one or more light sources and directing this light at leastonce through a sample of the gas prior to the light reaching a detectorfor quantifying absorbance, fluorescence or other emission, or the likeresulting from interaction of the light with components of the gassample.

In some applications, a gas volume or flowing gas stream being analyzedby a spectrometer can include chemically reactive compounds, which havethe potential to interact in undesirable ways with various components ofthe spectrometer cell. In particular, optical materials and the opticalcoatings often used with optical materials in creating windows ormirrors for passing or reflecting light into a sample gas can be quitesensitive to contamination. As an example, optical materials and theiroptical coatings may be exposed to one or more of acidic and basicgases, liquids, chlorinated compounds, fluorinated compounds, highmolecular weight compounds (which therefore typically have relativelylow vapor pressures and a tendency to condense onto surfaces), etc.,which can alter, influence, or otherwise affect the optical performanceof such materials, potentially resulting in false or otherwise flawedspectroscopic analyses.

In addition, currently available sample cell configurations for use inspectrometers are generally not compatible with applications requiringoperation over a wide range of temperatures. In particular withmulti-pass sample cells that include one or more reflective opticalelements, such as for example mirrors or the like, positioned to causeone or more beams of light generated by one or more light sources to bereflected within the sample cell (e.g. to increase a path length overwhich the one or more light beams travel within a gas sample containedwithin the sample cell), changes in temperature can require realignmentof the reflective optical elements at the operating conditions.

SUMMARY

The current subject matter can provide various advantages overpreviously available approaches to construction of a spectrometer cell.In one aspect, a spectrometer cell having an internal volume forcontaining a sample gas includes a spacer, an end cap, and a mirror. Thespacer at least partially defines the internal volume and includes afirst contact end. The end cap is positioned proximate to the firstcontact end and also at least partially encloses the internal volume.The mirror includes a reflective surface for receiving and redirecting abeam of light at least once along an optical path length that originatesfrom at least one light source. The optical path length passes at leastonce through the internal volume. The mirror is secured in place by amechanical attachment that includes attachment materials that arechemically inert to at least one reactive gas compound. The mechanicalattachment holds the optical axis in a fixed orientation relative toother components of the spectrometer cell and of a spectrometer devicethat comprises the spectrometer cell.

In another aspect, a spectrometer includes an end cap that includes aninner face recessed within a spacer structure. The spacer structure andthe inner face at least partially define the internal volume of thespectrometer cell. A mirror can include a reflective surface forreceiving and redirecting a beam of light at least once along an opticalpath length that originates from at least one light source. Thereflective surface has an optical axis, and the optical path lengthpasses at least once through the internal volume. The mirror is securedin place on the inner face by a mechanical attachment that includesattachment materials that are chemically inert to at least one reactivegas compound. The mechanical attachment holds the optical axis in afixed orientation relative to other components of the spectrometer celland of a spectrometer device that comprises the spectrometer cell.

In another interrelated aspect, a method includes defining, at leastpartially, an internal volume of a spectrometer cell with a spacer,further enclosing the internal volume with an end cap connected to thefirst contact end, and receiving and redirecting a beam of light atleast once along an optical path length that originates from at leastone light source. The spacer includes a first contact end. The opticalpath length passes at least once through the internal volume. Thereceiving and redirecting occurs at a mirror that includes a reflectivesurface having an optical axis and being secured in place by amechanical attachment that includes attachment materials that arechemically inert to at least one reactive gas compound. The mechanicalattachment holds the optical axis in a fixed orientation relative toother components of the spectrometer cell and of a spectrometer devicethat comprises the spectrometer cell.

In some variations one or more of the following can optionally beincluded in any feasible combination. The reflective surface can includemultiple (e.g. two or more) reflective features, which can havedifferent curvatures. The spacer piece and at least one of the end capand the mirror can have a similar thermal expansion coefficient. Thereflective surface can be formed of a material including at least one ofstainless and ceramic. The mirror can further include one or moreadditional reflective coatings on the reflective surface. The at leastone reactive gas compound can include at least one of an acid gascompound, a basic gas compound, a fluorinated compound, and achlorinated gas compound. The reflective surface can have a surfaceroughness in one or more of the following ranges: less thanapproximately 10 Å rms, approximately 10 Å rms to approximately 25 Årms, approximately 25 Å rms to approximately 50 Å rms, approximately 50Å rms to approximately 100 Å rms, approximately 100 Å rms toapproximately 250 Å rms, and approximately 250 Å rms to approximately500 Å rms. The reflective surface can include at least one of a planarsurface, a spherical curvature, and a parabolic curvature and optionallytwo or more of such surface shapes.

The reflective surface can be integral to an inner face of the end cap.The inner face can be directed inward toward the internal volume. Themechanical attachment can include a direct and stable physical contactbetween the end cap and the spacer secured by at least one attachmentdevice. The direct and stable physical contact can ensure at least oneof a reproducible alignment and a reproducible orientation of theoptical axis relative to the beam of light when the spacer and the endcap are assembled.

The reflective surface can alternatively be disposed on a detachablemirror part. The detachable mirror part can be mechanically connectableto a face of the end cap. The detachable part and an inner face of theend cap can have mating reference surfaces that ensure at least one of aspecific alignment and a specific orientation of the optical axisrelative to the beam of light when the detachable mirror part and theend cap are joined and the end cap is assembled to the spacer and adirect and stable physical contact between the end cap and the spacer issecured by at least one attachment device.

An inner spacer can be disposed within the internal volume. The innerspacer can have a contact end. The mirror can include a mirror piecethat is not directly attached to the end cap. The mirror piece caninclude a front contact surface on a same side of the mirror piece asthe reflective surface and a rear contact surface opposite the frontcontact surface. The mirror piece can be disposed proximate an innerface of the end cap such that the inner face contacts the rear contactsurface and the contact end of the inner spacer contacts the frontcontact surface to thereby hold the mirror piece secure such that atleast one of a reproducible alignment and a reproducible orientation ofthe optical axis relative to the beam of light are ensured when thespacer, the inner spacer, the mirror piece, and the end cap areassembled.

An apparatus, which can in some implementations be a spectrometer, suchas for example a tunable diode laser absorption spectrometer, caninclude a spectrometer cell including one or more of the featuresdescribed herein. Such an apparatus can optionally further include oneor more light sources for generating one or more light beams, a detectorthat quantifies a received intensity of light emitted from the lightsource along a path length, and at least one processor that performsoperations comprising controlling a driving current to the laser sourceand receiving intensity data from the detector. The at least oneprocessor can optionally cause the laser source to provide light havinga wavelength modulation frequency and can demodulate the intensity datareceived from the detector to perform a harmonic spectroscopy analysismethod. The at least one processor can mathematically correct ameasurement spectrum to account for absorption by compounds in a samplegas through which the light passes. In some examples, the mathematicalcorrection can include subtraction of a reference spectrum from themeasurement spectrum where the reference spectrum is collected for asample of the sample gas in which a concentration of a target analytehas been reduced, for example as part of a differential absorptionspectroscopy approach.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the description, help explain someof the principles associated with the disclosed implementations. In thedrawings,

FIG. 1A and FIG. 1B show diagrams illustrating a cross sectional and apartial cutaway elevation view, respectively, of an example of amulti-pass spectrometer cell;

FIG. 2 shows a diagram illustrating a partial cutaway elevation view ofan example of a multi-pass spectrometer cell consistent withimplementations of the current subject matter;

FIG. 3A and FIG. 3B show diagrams illustrating a top and a side view,respectively, of end caps usable in a spectrometer cell consistent withimplementations of the current subject matter;

FIG. 4 shows a diagram illustrating a side view of a spectrometer cellconsistent with implementations of the current subject matter; and

FIG. 5 is a process flow diagram illustrating aspects of a method havingone or more features consistent with implementations of the currentsubject matter.

When practical, similar reference numbers denote similar structures,features, or elements.

DETAILED DESCRIPTION

One example of a multi-pass spectrometer cell is a Herriott cell, inwhich a mirror is positioned at each end of a cylindrical or otherwiseclosed gas volume. Such mirrors include a reflective surface forreceiving and redirecting a beam of light at least once along an opticalpath length that originates from one or more light sources. FIG. 1A andFIG. 1B respectively show a cross-section and a partially cutawayelevation view of an example Herriott cell 100. One or more beams oflight 102 generated by the one or more light sources (not shown) can bedelivered to the space between the two mirrors 104,106, for examplethrough a hole, window, or port 110 in one of the mirrors 104. The oneor more beams of light can be reflected multiple times, optionally atleast once, between the mirrors 104, 106 such that an extendedspectroscopic path length is created within a relatively compact volumeof gas. The two mirrors can be held at a desired distance from eachother by one or more structural components of the spectrometer cell 100.For example, as shown in FIG. 1A, a spacer 112 can be disposed toprovide a structural support between the two mirrors 104, 106. Theelevation view of FIG. 1B shows the Herriot cell 100 with the spacer 112removed such that the complex path length of the one or more light beamsreflecting between two cylindrical mirrors 104, 106 is visible.

A conventional mirror for use in a spectrometer cell, including but notlimited to a Herriott cell, can suffer from difficulties in keeping themirror surface clean, for example in an analytical environment in whichprocess upsets, background gas contamination, or the like can occur.Additionally, as noted above, conventional mirrors, which are generallymade of glass, often have a different thermal expansion coefficientrelative to other materials (e.g. stainless steel, aluminum, othermetals, ceramics, or the like), which are commonly used for formingstructural elements (e.g. the spacer 112) of a spectrometer cell. Amirror having different thermal expansion properties compared to thestructural components of a spectrometer cell can lead to opticalmisalignment issues if the spectroscopic cell is used at an operatingtemperature that differs significantly from a temperature at which thespectrometer cell was assembled and originally aligned. Additionally,because of the relative fragility of glass and glass-like materials, aglass mirror generally cannot be directly mechanically attached (forexample using screws, bolts, compression fittings, mechanical clamps, orthe like) to structural or flow path parts of a spectrometer cell.Rather, in conventional spectrometers, a glass mirror is typicallymounted to a support formed out of a structural material, such as metalor another material, using a flexible adhesive material, such as forexample room temperature vulcanizing (RTV) silicone. The support is thenmechanically attached to other structural features of the spectrometercell and the alignment of the mirror with other optical components, alight source, a detector, etc. of the spectrometer is set, usually underfactory conditions with very exacting tolerances.

Many flexible adhesives typically used in spectrometer cells are notcompatible with one or more reactive gas mixtures. For example,chlorinated compounds present in a gas sample can tend to chemicallyattack RTV silicone. The resulting reactions can have undesirableeffects in either weakening the RTV silicone, thereby potentiallycausing a loss of structural integrity of the mirror mounting, orreleasing gas-phase reaction by-products into the gas sample beinganalyzed, thereby potentially altering a spectrometer reading relativeto the actual composition of the gas sample absent such contamination.Flexible adhesives (e.g. RTV silicone) generally used in mirror mountingin conventional spectrometers can also present difficulties with hightemperature operation (e.g. above approximately 120° C., aboveapproximately 200° C., etc.) in that these materials generally have poorthermal stability at elevated temperatures.

Elevated operating temperatures can be important for analyticalapplications in streams containing ammonia (NH₃), hydrogen sulfide (H₂S)and other compounds of potential interest. In sulfur recovery unitemissions control, for example, the gas stream can typically be heldabove at least 120° C. to prevent formation of sulfides, which wouldartificially which would falsify the emissions measurement (per U.S.Environmental Protection Agency regulations). Other potential hightemperature spectroscopy applications can include H₂S measurement in aClaus unit, which removes sulfur compounds from petrochemical streams.Gas streams in such processes can typically be at temperatures in excessof approximately 180° C. to prevent condensation and sublimation ofelemental sulfur. Quantification of motor emissions on motor test bedsare another potential application of high temperature spectroscopy forwhich implementations of the current subject matter can resolvepotential issues with conventional, currently available spectroscopicapproaches. With ever tightening emissions reduction requirements,tunable diode laser (TDL) spectrometric analyzers can be being used toanalyze motor exhaust. Due to the temperature constraints ofconventional spectrometers, the exhaust gas typically has to be cooleddown prior to analysis. A high temperature sample cell TDL measurementcan be used to improve the quantitative measurement, avoidinguncertainties related to dissolution of emitted gases in condensedwater, which can be a large component of internal combustion engineexhaust.

Implementations of the current subject matter can resolve one or moreweaknesses in currently available spectrometer cell configurations andapproaches, for example by providing a spectrometer cell usable in aspectrometer that can eliminate or at least reduce the effects ofchemically reactive compounds on mirrors and elements of thespectrometer cell associated with mounting and securing of such mirrorsto other components of the spectrometer cell. This effect can beachieved through the use of a mechanical attachment that secures themirror in place using attachment materials that are chemically inert toat least one reactive gas compound. In various examples, the at leastone reactive gas compound can include at least one of an acid gascompound, a basic gas compound, a fluorinated compound, and achlorinated gas compound. Other reactive gas compounds can also be ofconcern, and the attachment materials can be inert to such otherreactive gas compounds as well.

In this manner, the occurrence of potentially false or otherwise flawedspectroscopic results can be mitigated, and a useful lifetime of aspectrometer can be extended. Additionally, mirrors can be constructedof materials that are closely matched in terms of thermal expansionproperties to spacers and other structural components of thespectrometer cell. Such a spectrometer cell can be effectively or atleast approximately athermal and may not require realignment of themirrors at any operating temperature or at least over a range ofexpected operating temperatures. A further advantage that can berealized in conjunction with implementations of the current subjectmatter can include an improved ability to clean a mirror of aspectrometer cell without requiring a factory recalibration of thespectrometer cell.

In some implementations of the current subject matter, a spectrometercell can include a reflective surface of a material that differs frommaterials conventionally used on optical components for spectrometersand other analytical equipment based on optical measurements. Mirrorsconsistent with implementations of the current subject matter can beformed out of one or more materials, which can advantageously bechemically resistant to attack by compounds expected to be present in asample gas. For example, instead of glass or the like, a reflector,referred to herein generically as a mirror, can be formed of a polishedmetal surface, a polished metal surface with one or more additionalreflective coatings applied thereto, a polished ceramic surface, apolished ceramic surface with one or more additional reflective coatingsapplied thereto, or the like.

Bulk materials that can be used in mirrors consistent withimplementations of the current subject matter can include stainlesssteel, for example 316 stainless steel or 320 stainless steel; ceramics,such as for example silica nitride, alumina, or the like; etc. Examplesof additional reflective coatings that can be applied consistent withimplementations of the current subject matter can include, but are notlimited to at least one of a metal and a dielectric material forenhancing reflectively of the mirror surface. Metals that can be used asreflective coatings can include gold, silver, aluminum, other metals,two or more metals in combination, and the like.

Because metals and ceramics and other materials consistent with theimplementations described generally can have mechanical propertiesconsistent with directly connecting components formed of such materialsto other structural components of a spectrometer cell, a mirror formedof one or more of these materials can be secured within a spectrometercell assembly without the need for a flexible adhesive, such as forexample RTV silicone or the like. In other words, the mechanicalattachment can secure the mirror in place such that an optical axis ofthe reflective surface (e.g. an axis that passes through the center ofcurvature of the reflective surface of the mirror and generallycoincides with the axis of rotational symmetry of the reflectivesurface) is held in a fixed and reproducible orientation relative toother components of the spectrometer cell and of a spectrometer devicethat comprises the spectrometer cell.

A directly mechanical attachment of a mirror component of a spectrometercell to one or more other structural components of the spectrometer cellcan be very useful for a variety of reasons. For example, a mirrormounted in a conventional manner using a flexible adhesive or othermounting approach that does not result in a positive solid on solidcontact can require fine tuning or other calibration and alignment in afactory or other controlled service setting to ensure that the one ormore beams of light are reflected along a desired axis to therebyimpinge upon the detector. The alignment process can be quite delicatebecause of the high tolerances required to properly align one or morebeams of light that can in some implementations be reflected numeroustimes before reaching the detector. As such, if a spectrometer cell isexposed to a process upset that results in deposition of one or morecondensed-phase (e.g. liquid, solid, adsorbed, chemisorbed, etc.)contaminants on a reflective surface of a mirror, or if the backgroundconstituents of a gas sample being analyzed contain compounds with asufficiently low vapor pressure or high reactivity that they tend toaccumulate on the mirror surfaces, cleaning of the mirror surfaces canbe a non-trivial exercise that often cannot be performed in the field.One example of this problem can arise in a spectrometer used inmonitoring natural gas, hydrocarbon streams, refining operations, or thelight can be exposed to a variety of relatively low vapor pressurecompounds, which can tend to condense on mirror surfaces.

An inability to clean a mirror in a spectrometer cell in the field (orotherwise without returning the entire spectrometer or spectrometer cellto a calibration facility can result in one or more of excessivedowntime while the spectrometer is removed from service to be cleanedand re-calibrated either at the factory or in another controlledsetting, less than optimal performance as the period between suchcleanings and re-calibrations is extended to reduce the fractionaldowntime of the analytical system, or the like.

Implementations of the current subject matter that include a mirror thatcan be directly and securely mechanically attached to one or more otherstructural components of a spectrometer cell can overcome thesedifficulties. Such a mirror can be removed from the spectrometer cell,cleaned using one or more approaches (e.g. wiping, polishing, immersionin one or more solvents, ultrasonic cleaning, or the like), and thenreturned to the spectrometer cell in the field. Because a direct andstable physical contact exists between such a mirror and the otherspectrometer cell components, concerns about alignment errors uponreinstallation of the removed mirror can be significantly reduced.

In another example of a spectrometer 200 shown in FIG. 2, one or moremirrors 104, 106 having one or more reflective mirror surfaces, whichcan be curved (e.g. spherical, parabolic, having a spatially varyingcurvature, etc.) or flat (e.g. planar), can be supported or anchored toone or more structural elements of a spectrometer cell 100. A reflectivesurface can optionally include two or more type of curvature, such asfor example a spherical mirror having one or more planar facets. FIG. 2shows a spectrometer 200 including a spectrometer cell 100 through whichlight 102 from a light source 202 is reflected multiple times betweentwo mirrors 104, 106 before reaching a detector 204. The light source202 and the detector can optionally be contained within a head unit 206,which can optionally include temperature control, electronics to controlthe light source (e.g. a laser) and receive signals from the detector,etc. A gas sample can flow into the spectrometer cell 100 via a gasinlet 208 and pass out of the spectrometer cell 100 via a gas outlet210. One or more of a temperature sensor 212, a pressure sensor 214, orother sensing devices (not shown) can be included to monitor conditionswithin the spectrometer cell 100.

FIG. 3A and FIG. 3B shows two approaches consistent with implementationsof the current subject matter relating to configurations of mirrors ormirror surfaces usable in a spectrometer cell 100. It will be understoodthat one or more mirrors used in a spectrometer cell 100 consistent withimplementations of the current subject matter can include one or morefeatures similar to those described and shown.

In FIG. 3A, which shows an elevation view of a spectrometer component300, a mirror 302 is formed as a contiguous part of an end cap 304 of aspectrometer cell (not shown in FIG. 3A). As shown in FIG. 3A, thereflective surface of the mirror 302 is integral to an inner face of theend cap 304. When assembled as part of a spectrometer cell, the innerface and therefore the reflective surface of the mirror 302 is directedinward toward the internal volume of the spectrometer cell. In thisexample, the end cap 304 includes machined holes 306 to accept boltsthat secure the end cap 304 to a cylindrical spacer component (not shownin FIG. 3A) that maintains a distance between the end cap 304 andanother component (which can optionally also include a mirror)positioned at an opposite end of a spectrometer cell. The spacer canalso serve to enclose a gas volume through which the light from thelight source passes one or more times. The end cap 304 can be machinedand then polished along a central axis to form a curved mirror surfacethat can thereby be aligned to extremely tight tolerances. The end cap304 is bolted or otherwise secured with one or more attachment devices(e.g. screws, clamps, etc.) directly to a solid mating surface on thespacer component of the spectrometer cell to form a mechanicalattachment that includes a direct and stable physical contact betweenthe end cap and the spacer. Accordingly, a fixed orientation of themirror optical axis can be reproducibly achieved even if the end cap 304is removed for cleaning, polishing, or other maintenance activities.

In FIG. 3B, which shows a side cross-sectional view of anotherspectrometer component 320, a separate, detachable mirror part 322 canbe removable from an end cap 324. The end cap 324 can be configured toreceive the detachable mirror part 322. The detachable mirror part 322and the end cap 324 can have mating reference surfaces that ensure atleast one of a specific alignment and a specific orientation of theoptical axis relative to the beam of light when the detachable mirrorpart 322 and the end cap 324 are assembled. In other words, and to makea positive and solid connection with the mirror 322 such that thealignment of the mirror 322 relative to the end cap 324 and to otherstructural components of the spectrometer cell (not shown in FIG. 3B) towhich the end cap 324 is mechanically secured using one or moreattachment devices can be maintained even if the end cap 324 is detachedfrom and reattached to the other structural components of thespectrometer cell or if the detachable mirror piece mirror 322 isremoved from and reattached to the end cap 324 on one face of the endcap 324. In the example shown in FIG. 3B, the end cap 324 includesmachined holes 306 to accept bolts that secure the end cap 324 to aspacer component (not shown in FIG. 3B) that maintains a distancebetween the end cap 324 and another component (which can optionally alsoinclude a mirror) positioned at an opposite end of a spectrometer cell.The mirror 322 can optionally be positioned in a recess machined intoone side or face of the end cap 324. The recess can advantageously bemachined to very tight tolerances such that the mirror fits snugly intothe recess. More generally, matching reference surfaces on the mirror322 and the face of the end cap 324 can be machined or otherwise formedto be capable of mating in a manner that ensures a specific alignmentand orientation of the mirror 322 relative to the beam of light emittedby the light source. A screw or other mechanical connection can beadded, for example through a screw or bolt hole 326 in the opposite sideof the end cap 324 from the recess.

The approach of FIG. 3B can in some examples facilitate ease ofmanufacture in that only the mirror 322 need be polished and have itsoptical axis aligned with high precision relative to one or morereference surfaces of the mirror 322 (e.g. the back of the mirror 322, acircumference of the mirror 322, etc.) while the end cap 324 need onlybe machined to the necessary tolerances to be joined with the mirror 322so that the optical axis is correctly aligned when the spectrometer cellis assembled. For cleaning or other maintenance, the end cap 324 can beremoved or otherwise detached other structural components of thespectrometer cell and then reinstalled without the need for additionalalignment procedures. Similarly, the mirror 322 can be removed orotherwise detached from the end cap 324. Because of the positivemechanical connection between one or more reference surfaces of themirror 322 and the end cap 324, accurate alignment can be achievedduring one or more disassembly and reassembly procedures without needfor a complicated realignment procedure.

In another example configuration that is consistent with one or moreimplementations of the current subject matter, an end cap can include aninner face recessed within a spacer structure. The spacer structure canbe integral to the end cap. In other words, the inner face canoptionally be formed as an end of a bore (e.g. a cylindrical bore) intoa solid structure such that a hollow area partially defining an internalvolume is formed by the walls of the bore and the end of the bore. Amirror or mirror piece consistent with one or more implementations ofthe current subject matter disclosed herein can be disposed at the endof the bore. For example, the end of the bore can be machined orotherwise formed to a desired reflector surface shape and polishedand/or coated with a reflective coating. Alternatively, a separate,detachable mirror piece can be added within the bore and secured at theend of the bore by a mechanical attachment. A mirror consistent withthis implementation can have one or more reflective surfaces forreceiving and redirecting a beam of light at least once along an opticalpath length that originates from at least one light source. The one ormore reflective surfaces can include at least one optical axis. Themechanical attachment can include attachment materials that arechemically inert to at least one reactive gas compound and can hold theoptical axis in a fixed orientation relative to other components of thespectrometer cell and of a spectrometer device that comprises thespectrometer cell.

In some examples, a mirror consistent with one or more implementationsof the current subject matter need not be polished to be as smooth as aglass mirror. For example, a mirror for use with a tunable diode laserspectrometer that relies upon harmonic spectroscopy techniques need notachieve as high a degree of reflectivity as a mirror used in a directabsorption method. The ability of a harmonic spectroscopy method tocompensate for losses of intensity of light emitted from the one or morelight sources can result from the fact that the metric for determining aconcentration of an analyte is a ratio of the harmonic signal to thedirect absorption signal. Attenuation of the light intensity due toreflectivity losses in the optical path generally affects both theharmonic signal in the direct signal equally. Therefore, a metal mirroras described herein can be ground to a surface roughness parameter thatis not necessarily smooth as is typically achieved with a glass mirror.For example, a surface roughness of a mirror consistent withimplementations of the current subject matter can optionally be in arange of approximately 10 Å rms (root mean squared) to 500 Å rms. Invarious implementations of the current subject matter, a surfaceroughness of the mirror can be approximately 500 Å rms, approximately250 Å rms, approximately 100 Å rms, approximately 50 Å rms,approximately 25 Å rms, approximately 10 Å rms, or any range includingtwo of these values as its outer limits. Other ranges of surfaceroughness are also within the scope of the current subject matter.

In some implementations of the current subject matter, a spectrometercell can be designed to be operable at temperatures in a range of 120°to 180° C. Such a spectrometer cell can be aligned at a firsttemperature, for example approximately when temperature, and thatalignment can remain consistent even at elevated temperatures. Incontrast, a conventional spectrometer cell that includes a glass mirrorand other structural components that are formed out of the material witha different thermal expansion coefficient than the glass material of themirror can experience unbalanced thermal expansion at the elevatedoperating temperature. This unbalanced thermal expansion and caused thealignment, which was established at the assembly temperature, to nolonger be valid at a elevated operating temperature. Particularly in aspectrometer cell in which one or more beams of light are reflectednumerous times between one or more mirrors or other reflective elements,a small deviation in the shape of the spectrometer so can result in thelight beam not impinging or only partially impinging upon the detector.This effect can cause the spectrometer to either not working all or workwith a reduced accuracy or potential to introduce significant errors. Toaddress such concerns, implementations of the current subject matter canoptionally include a spectrometer cell in which all of the components,including both the spacer and one or more end caps, are formed of thesame material or if of different materials of materials with comparablethermal expansion coefficients. A single material used in thesecomponents can in some examples be stainless steel, optional includingone or more alloys, a ceramic, or other comparable materials. Stainlesssteel can be particularly advantageous in some implementations becausestainless steel is a convenient material for use in making things andother connections to gas tubing that may be used to deliver a gas sampleto the volume of the spectrometer cell and to allow such a sample to bevented from the spectrometer cell.

In another, related implementation of the current subject matter, anathermal spectrometer cell can be formed of a two-layer structure, forexample as illustrated in FIG. 4. As shown in FIG. 4, consistent withsome implementations of the current subject matter, a spectrometer cell400 can include an outer structure and an inner structure. The outerstructure can be formed of one or more structural materials, such as forexample stainless steel, ceramic, or the like. As shown in FIG. 4, theouter structure includes at least one, and optionally two, end caps 402and an outer spacer 404. Gas connections 406, physical support andattachment points, and the like can attach to the outer spacer piece404. The end caps 402 and the outer spacer piece 404 can be formed of astructurally stable material, such as for example stainless steel,aluminum, ceramic, etc. The inner structure can include one or moremirror pieces 410, which can be positioned near one or both of the endcaps 402. The one or more mirror pieces can include a front contactsurface on a same side of the mirror piece as a reflective surface and arear contact surface opposite the front contact surface.

An inner spacer 412 can be included and can have a contact end (or twocontact ends as shown in FIG. 4) respectively in contact with one of theone or more mirror pieces 410. In one example, the inner spacer 412 andthe one or more mirror pieces 410 can all be formed of a same material,such as for example glass. One or both of the end caps 402 can includeat least one optical pass-through to allow entry and/or exit of a lightbeam as it passes between a light source and a detector. A mirror piece410 can be disposed proximate an inner face of the corresponding end cap402 such that the inner face contacts the rear contact surface and thecontact end of the inner spacer contacts the front contact surface tothereby hold the mirror piece secure such that at least one of areproducible alignment and a reproducible orientation of the opticalaxis of the reflective surface of the mirror relative to the beam oflight are ensured when the outer spacer 404, the inner spacer 412, theone or more mirror pieces 410, and the one or more end caps 402 areassembled.

The one or more mirror pieces 410 can optionally be circular and caninclude an annular registration surface around the outer circular edges.The inner spacer 412 can be cylindrical in shape, with a contact edgelocated on at least one end of the cylinder for mating with the annularregistration surface of a mirror piece 410. In this manner, the one ormore mirror pieces 410 can be machined or otherwise formed to a closetolerance and mechanically held in contact with one or more rigid, solidcontact surfaces on the inner spacer piece such that alignment of theone or more mirror pieces 410 is ensured through this positivemechanical contact and a flexible adhesive or other securing material isnot required. The parts that assemble to create the inner structure canbe held in contact with each other (e.g. the annular registrationsurface of the mirror piece 410 can be held in physical contact with thecontact edge located on at least one end of the inner spacer 412) by theouter structure. In an example in which the outer structure is formed ofa metal, ceramic, or other material that can be bolted, screwed, orotherwise mechanically connected together, this outer structure canencapsulate, protect, support, and retain the one or more mirror pieces410 and the inner spacer 412 in contact with each other. Forming of theinner structure from a single type of material or two or more materialshaving a same thermal or similar expansion coefficient can alleviateissues discussed above in regards to misalignment of the optical pathcaused by mismatched expansion or contraction under changes in operatingtemperature of the spectrometer cell.

The inner spacer 412 can, consistent with implementations of the currentsubject matter, be a cylinder or other shape with a volume at leastpartially enclosed inside the hollow part of the shape. In otherimplementations, the inner spacer 412 can be a rod or other solid shapehaving contact ends that contact with a contact area on each of one ormore mirror pieces 410 to create a positive mechanical connection. Insome implementations of the current subject matter, the inner space 412can include two or more pieces that act to stabilize and secure the oneor more mirror pieces. For example, an inner spacer 412 can assume atripod-like configuration, in which three (or two or optionally morethan three) rods, each having a contact end that creates a positivemechanical connection with the a contact area on each of one or moremirror pieces 410.

FIG. 5 shows a process flow chart 500 that illustrates features of amethod. Ne or more of these features can be present in variousimplementations of the current subject matter. At 502, a spacer thatincludes a first contact end at least partially defines an internalvolume of a spectrometer cell is defined. At 504, the internal volume isfurther enclosed with an end cap connected to the first contact end, andat 506 a beam of light is received and redirected at least once along anoptical path length that originates from at least one light source. Theoptical path length passes at least once through the internal volume.The receiving and redirecting occurs at a mirror that includes areflective surface having an optical axis and being secured in place bya mechanical attachment comprising attachment materials that arechemically inert to at least one reactive gas compound. The mechanicalattachment holds the optical axis in a fixed orientation relative toother components of the spectrometer cell and of a spectrometer devicethat comprises the spectrometer cell.

The subject matter described herein can be embodied in systems,apparatus, methods, and/or articles depending on the desiredconfiguration. The implementations set forth in the foregoingdescription do not represent all implementations consistent with thesubject matter described herein. Instead, they are merely some examplesconsistent with aspects related to the described subject matter.Although a few variations have been described in detail above, othermodifications or additions are possible. In particular, further featuresand/or variations can be provided in addition to those set forth herein.For example, the implementations described above can be directed tovarious combinations and subcombinations of the disclosed featuresand/or combinations and subcombinations of several further featuresdisclosed above. The specific configuration of a spectrometer cell cantake any of a large number of shapes consistent with the subject matteras claimed below. In addition, the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. Other implementations may be within the scope of the followingclaim.

1. A spectrometer cell having an internal volume for containing a samplegas, the spectrometer cell comprising: a spacer, the spacer at leastpartially defining the internal volume and comprising a first contactend; an end cap, the end cap positioned proximate to and contacting thefirst contact end and at least partially enclosing the internal volume;and a mirror, the mirror comprising a reflective surface for receivingand redirecting a beam of light at least once along an optical pathlength that originates from at least one light source, the reflectivesurface having an optical axis, the optical path length passing at leastonce through the internal volume, the mirror being secured in place by amechanical attachment comprising attachment materials that arechemically inert to at least one reactive gas compound, the mechanicalattachment holding the optical axis in a fixed orientation relative toother components of the spectrometer cell and of a spectrometer devicethat comprises the spectrometer cell.
 2. A spectrometer cell as in claim1, wherein the spacer piece and at least one of the end cap and themirror have a similar thermal expansion coefficient.
 3. A spectrometercell as in claim 1, wherein the reflective surface is formed of amaterial comprising at least one of stainless steel and ceramic.
 4. Aspectrometer cell as in claim 1, wherein the mirror further comprisesone or more additional reflective coatings on the reflective surface. 5.A spectrometer cell as in claim 1, wherein the at least one reactive gascompound comprises at least one of an acid gas compound, a basic gascompound, a fluorinated compound, and a chlorinated gas compound.
 6. Aspectrometer cell as in claim 1, wherein the reflective surface has asurface roughness in one or more of the following ranges: less thanapproximately 10 Å rms, approximately 10 Å rms to approximately 25 Årms, approximately 25 Å rms to approximately 50 Å rms, approximately 50Å rms to approximately 100 Å rms, approximately 100 Å rms toapproximately 250 Å rms, and approximately 250 Å rms to approximately500 Å rms.
 7. A spectrometer cell as in claim 1, wherein the reflectivesurface comprises at least one of a planar surface, a sphericalcurvature, and a parabolic curvature.
 8. A spectrometer cell as in claim1, wherein the reflective surface is integral to an inner face of theend cap, the inner face being directed inward toward the internalvolume, and wherein the mechanical attachment comprises a direct andstable physical contact between the end cap and the spacer secured by atleast one attachment device, the direct and stable physical contactensuring at least one of a reproducible alignment and a reproducibleorientation of the optical axis relative to the beam of light when thespacer and the end cap are assembled.
 9. A spectrometer cell as in claim1, wherein the reflective surface is disposed on a detachable mirrorpart, the detachable mirror part being mechanically connectable to aface of the end cap, the detachable part and an inner face of the endcap having mating reference surfaces that ensure at least one of aspecific alignment and a specific orientation of the optical axisrelative to the beam of light when the detachable mirror part and theend cap are joined and the end cap is assembled to the spacer and adirect and stable physical contact between the end cap and the spacer issecured by at least one attachment device.
 10. An spectrometer cell asin claim 1, further comprising an inner spacer disposed within theinternal volume, the inner spacer having a contact end, wherein themirror comprises a mirror piece that is not directly attached to the endcap, the mirror piece comprising a front contact surface on a same sideof the mirror piece as the reflective surface and a rear contact surfaceopposite the front contact surface, the mirror piece being disposedproximate an inner face of the end cap such that the inner face contactsthe rear contact surface and the contact end of the inner spacercontacts the front contact surface to thereby hold the mirror piecesecure such that at least one of a reproducible alignment and areproducible orientation of the optical axis relative to the beam oflight are ensured when the spacer, the inner spacer, the mirror piece,and the end cap are assembled.
 11. A method comprising: defining, atleast partially, an internal volume of a spectrometer cell with aspacer, the spacer comprising a first contact end; further enclosing theinternal volume with an end cap connected to the first contact end; andreceiving and redirecting a beam of light at least once along an opticalpath length that originates from at least one light source, the opticalpath length passing at least once through the internal volume, thereceiving and redirecting occurring at a mirror comprising a reflectivesurface, the reflective surface having an optical axis and being securedin place by a mechanical attachment comprising attachment materials thatare chemically inert to at least one reactive gas compound, themechanical attachment holding the optical axis in a fixed orientationrelative to other components of the spectrometer cell and of aspectrometer device that comprises the spectrometer cell.
 12. A methodas in claim 11, wherein the spacer piece and at least one of the end capand the mirror have a similar thermal expansion coefficient.
 13. Amethod as in claim 11, wherein the reflective surface is formed of amaterial comprising at least one of stainless steel and ceramic.
 14. Amethod as in claim 11, wherein the mirror further comprises one or moreadditional reflective coatings on the reflective surface.
 15. A methodas in claim 12, wherein the at least one reactive gas compound comprisesat least one of an acid gas compound, a basic gas compound, afluorinated compound, and a chlorinated gas compound.
 16. A method as inclaim 11, wherein the reflective surface has a surface roughness in oneor more of the following ranges: less than approximately 10 Å rms,approximately 10 Å rms to approximately 25 Å rms, approximately 25 Å rmsto approximately 50 Å rms, approximately 50 Å rms to approximately 100 Årms, approximately 100 Å rms to approximately 250 Å rms, andapproximately 250 Å rms to approximately 500 Å rms.
 17. A method as inclaim 11, wherein the reflective surface comprises at least one of aplanar surface, a spherical curvature, and a parabolic curvature.
 18. Amethod as in claim 11, wherein the reflective surface is integral to aninner face of the end cap, the inner face being directed inward towardthe internal volume, and wherein the mechanical attachment comprises adirect and stable physical contact between the end cap and the spacersecured by at least one attachment device, the direct and stablephysical contact ensuring at least one of a reproducible alignment and areproducible orientation of the optical axis relative to the beam oflight when the spacer and the end cap are assembled.
 19. A method as inclaim 11, wherein the reflective surface is disposed on a detachablemirror part, the detachable mirror part being mechanically connectableto a face of the end cap, the detachable part and an inner face of theend cap having mating reference surfaces that ensure at least one of aspecific alignment and a specific orientation of the optical axisrelative to the beam of light when the detachable mirror part and theend cap are joined and the end cap is assembled to the spacer and adirect and stable physical contact between the end cap and the spacer issecured by at least one attachment device.
 20. An spectrometer cell asin claim 1, further comprising an inner spacer disposed within theinternal volume, the inner spacer having a contact end, wherein themirror comprises a mirror piece that is not directly attached to the endcap, the mirror piece comprising a front contact surface on a same sideof the mirror piece as the reflective surface and a rear contact surfaceopposite the front contact surface, the mirror piece being disposedproximate an inner face of the end cap such that the inner face contactsthe rear contact surface and the contact end of the inner spacercontacts the front contact surface to thereby hold the mirror piecesecure such that at least one of a reproducible alignment and areproducible orientation of the optical axis relative to the beam oflight are ensured when the spacer, the inner spacer, the mirror piece,and the end cap are assembled.
 21. A spectrometer cell having aninternal volume for containing a sample gas, the spectrometer cellcomprising: an end cap, comprising an inner face recessed within aspacer structure, the spacer structure and the inner face at leastpartially defining the internal volume a mirror, the mirror comprising areflective surface for receiving and redirecting a beam of light atleast once along an optical path length that originates from at leastone light source, the reflective surface having an optical axis, theoptical path length passing at least once through the internal volume,the mirror being secured in place on or proximate to the inner face by amechanical attachment comprising attachment materials that arechemically inert to at least one reactive gas compound, the mechanicalattachment holding the optical axis in a fixed orientation relative toother components of the spectrometer cell and of a spectrometer devicethat comprises the spectrometer cell.
 22. A spectrometer cell as inclaim 1, wherein the mechanical attachment comprises a direct and stablephysical contact such that the fixed orientation is reproducible whenthe mirror is removed from and replaced into the spectrometer cell. 23.A spectrometer cell as in claim 1, wherein the direct and stablemechanical attachment does not include a flexible adhesive.